Like icons wallpapers are easily customizable and add a good effect to your PC. When you start you windows, the first thing you see after logging in is wallpaper so make sure that you have a smooth wallpaper with vibrant colors and soothing effect. A much brighter wallpaper can cause strain on your eyes and head. Usually wallpaper reflect the personality and mood of user. I usually use soft wallpapers of video game characters which are elegant, not too bright and have a pleasant look.
Here are links for some of the sites which have a quite decent collection of wall papers. Happy Surfing.